Three hours ago, Mark Engelberg wrote:
> On Thu, Jul 14, 2011 at 4:35 AM, Thomas Chust <chust at web.de> wrote:
> > While Clojure *mitigates* the problems of non-hygienic macros
> > using namespaces and a shorthand syntax for freshly generated
> > identifiers, it doesn't *solve* the problems. Racket's macro
> > system, on the other hand, does solve the problems and since that
> > involves some heavy lifting, it may seem more complicated at first
> > glance.
>> I would like to better understand how Clojure's mitigation strategy
> is insufficient. Since Eli's document is all about the while macro,
> let's look at Clojure's while macro.
I know very little about clojure, so I don't have a clue how this code
works. But one thing that seems fishy: I used your code as is, but
before entering it, I entered
(defmacro when [test & body]
`(if ~test ((fn [] (println "in my when") [email protected]))))
and it got into an infinite loop (and printed "in my when").
(I couldn't figure out clojure's `begin' equivalent, so I did it the
stupid way). I tried to read around and figure out why that happens,
but I still don't see the problem.
In any case, it looks like clojure macros are closer to racket than
they are to CL -- with ` being roughly similar to #` (it's even called
"syntax-quote"). So I also don't know whether there's something that
is misbehaved there. On thing that I did try is:
(defmacro myif [x y z] `(if ~x ~y ~z))
(defmacro myor [x y] `(let [t# ~x] (myif t# t# ~y)))
(let [myif ...] (myor ...))
and that doesn't break too -- it *looks* like `myif' is a plain symbol
when expanding the last expression, which somehow gets qualified by a
namespace later. Based on this, I tried to dig more with local
macros, and I found out that clojure doesn't have them. Whatever
clojure does, I think that prohibiting local macros is important. (I
see that there's some library with it, but that goes well beyond my
interest/tiredness threshold.) Either that, or there's some way to
have "uninterned namespaces".
--
((lambda (x) (x x)) (lambda (x) (x x))) Eli Barzilay:
http://barzilay.org/ Maze is Life!